Efficient irrigation is a design objective of many different types of irrigation devices, such as gear-drive rotors, rotary spray nozzles, and fixed spray nozzles. That objective has been heightening due to concerns at the federal, state and local levels of government regarding the efficient usage of water. Over time, irrigation devices have become more efficient at using water in response to these concerns. However, those concerns are ongoing as demand for water increases.
As typical irrigation sprinkler devices project streams or sprays of water from a central location, there is inherently a variance in the amount of water that is projected to areas around the location of the device. For example, there may be a greater amount of water deposited further from the device than closer to the device. This can be disadvantageous because it means that some of the area to be watered will be over watered and some of the area to be watered will receive the desired about of water or, conversely, some of the area to be watered will receive the desired amount of water and some will receive less than the desired about of water. In other words, the distribution of water from a single device is often not uniform.
One measure of how uniformly water is applied to an area being watered is called Distribution Uniformity “DU”, which is expressed as a percentage. One common measure of Distribution Uniformity is the Lower Quarter Distribution Uniformity (“DUlq”), which is a measure of the average of the lowest quarter of samples, divided by the average of all samples:
      DU    lq    =            Average      ⁢                          ⁢      Catch      ⁢                          ⁢      of      ⁢                          ⁢      Lower      ⁢                          ⁢      Quarter      ×      100              Average      ⁢                          ⁢      Catch      ⁢                          ⁢      Overall      For example, if all samples are equal, the DU is 100%. If a proportion of the area greater than 25% receives zero application the DU will be 0%. DU can be used to determine the total watering requirement during irrigation scheduling. For example, one may want to apply not less than one inch of water to the area being watered. If the DU were 75%, then the total amount to be applied would be the desired about of water (one inch) divided by the DU (75%), or 1.33 inches of water would be required so that only a very small area receives less than one inch of water. The lower the DU, the less efficient the distribution and the more water that must be applied to meet the minimum desired. This can result in undesirable over watering in one area in order to ensure that another area receives the minimum water desired.
Another measurement is called the Scheduling Coefficient (“SC”). Unlike the DU, the scheduling coefficient does not measure average uniformity. Instead, it is a direct indication of the dryness of the driest turf areas (critical areas). The measurement is called the Scheduling Coefficient because it can play a role in establishing irrigation times. It is based on the critical area to be watered. To calculate the SC, one first identifies the critical area in the water application pattern which is receiving the least amount of water. The amount of water applied to this critical area is divided into the average amount of water applied throughout the irrigated area to obtain the Schedule Coefficient. The scheduling coefficient indicates the amount of extra watering needed to adequately irrigate the critical area. If perfect uniformity were obtained, the scheduling coefficient would be 1.0 (no extra watering needed to adequately irrigate the critical area). By way of example, assume that an irrigation pattern has a scheduling coefficient of 1.8. After 15 minutes of irrigation, a critical area would still be under-watered due to non-uniformity. It will take an additional 12 minutes (15 minutes×1.8) to apply an adequate amount of water to the critical area (or 27 minutes total). While that is the amount of time needed to water the critical area, the result is that other areas will be over-watered.
There are many applications where conventional spray nozzle irrigation devices are desirable for use. Unfortunately, conventional spray nozzle irrigation devices can undesirably have lower DUlq values. For example, some conventional fixed spray devices can have DUlq values of about 65% and be considered to have a very good rating, DUlq values of about 70% for rotors are considered to have a very good rating.